CHICAGO AND VICINITT. :: I 



nesquii, etc. The entire bluff flora down to the river's edge is xero- 

 phytic, except in shaded situations. 



When a stream in its meanderings ceases to erode at the base of a 

 bluff, increased opportunity is given for plant life. Through surface 

 wash the slopes become more and more gentle. Mesojjhytic vegeta- 



FlG. 7.— Ravine in the Niagara limestone at Loclcport, showing the beginnings of a flood 

 plain. 



tion comes in at the foot of the bluff and creeps up as the slopes 

 decrease. Finally the xerophytes are driven from their last strong- 

 hold, the top of the slope, and. the mesophytes have come to stay,, at 

 least until the river returns and enters upon another stage of cliff 

 erosion. The growth of a ravine into a valley with xerophytic bluffs is. 

 rapid, when expressed in terms of geology, but far less rapid when 

 expressed in terms of vegetation. A ravine in the vigor of youth may 

 develop so slowly that forest trees may grow to a considerable size 

 without any perceptible change in the erectness of their trunks. Thus, 



